1. Field of the Invention
This invention relates to an internal combustion engine with a turbo supercharger provided with a turbine driven by exhaust gas, and a compressor connected with the turbine by a common shaft. The compressor is driven by the turbine for compressing intake air to provide a supercharging effect on the intake gas, more specifically to an engine with a primary turbo supercharger operated mainly and a secondary turbo supercharger operated in a specific engine operating condition.
2. Description of the Prior Art
Conventionally, there has been known an internal combustion engine with a primary and secondary turbo superchargers wherein a control for the superchargers is switched between a first condition, in which one of them is driven, and a second condition, in which both of them are driven, in accordance with an engine operating condition, as disclosed in Japanese Utility Model Public Disclosure No. 60-178329. Japanese Patent Public disclosure Nos. 56-41417, 59-160022, 60-259722 (corresponding to European patent application No. 166869) and Japanese Utility Model Publication No. 57-12177 disclose similar types of engines, respectively. The engine disclosed in Japanese Utility Model Disclosure No. 60-178329 is provided with a plurality of cylinders having respective exhaust passages therefrom. The exhaust passages are divided into two groups. There are provided gate valves for controlling introduction of the exhaust gas into the turbo superchargers. Under the first condition, the exhaust gas from the cylinders are introduced into only the primary turbo supercharger through one of the groups of the exhaust passages by virtue of the operation of the gate valves. In the second condition, the exhaust gas is introduced into both the primary and secondary turbo superchargers. Such engines as disclosed in the Japanese publication can improve the efficiency of supercharging effect in accordance with the engine operating condition.
Meanwhile, in order to get a better performance of the turbo supercharger, it is critical that the superchargers are kept in an appropriate thermal range.
It should, however, be noted that it is difficult for the conventional internal combustion engine with a plurality of turbo superchargers to provide all of the turbo superchargers with a substantially uniform, thus proper, thermal condition while operating. This is because an operating conditions is different in each supercharger, for instance, the primary turbo supercharger which is driven in any supercharging condition of the engine, tends to be heated because of a heavy load in operating period, as compared with the secondary turbo supercharger, which is driven in only a specific engine operating condition. The turbo superchargers are usually arranged to be subjected to an air cooling effect by a cooling fan and blowing air into an engine room while the vehicle is running, in addition to a cooling effect provided by a coolant. As a result, some of the turbo superchargers are kept at an unduly high temperature and some of them at an unduly low temperature. This may cause a deterioration the warming up property of the superchargers.